Evolution of species range limits. Takuji Usui (Angert Lab) BIOL Nov 2017

Transcription

1 Evolution of species range limits Takuji Usui (Angert Lab) BIOL Nov 2017

2 Range limits often occur on continuous ecological gradients Range maps modified from Sheth et al. (2014). J Biogeogr., 41, Mimulus photo credit see end

3 A fundamental evolutionary question With enough time for mutation and selection to act, when and why does adaptation fail at the range margin?

4 A fundamental evolutionary question With enough time for mutation and selection to act, when and why does adaptation fail at the range margin? (1) Populations typically smaller and more fragmented towards margin Allee effects Greater genetic drift Less in situ mutations Center Margin (2) Populations act as sink to central populations (swamped by gene flow) Haldane (1956) Proc R Soc Lond B Biol Sci 145,

5 Theoretical models of range limits Models at different types of range margins 1. Abiotic environment 2. Competitive margins 3. Hybridization at margins

6 Modelling selection and gene flow Kirkpatrick & Barton, 1997 Populations along a continuous 1D environmental gradient Density-dependent population growth Random dispersal Kirkpatrick & Barton (1997). Am Nat., 150, 1-23

7 Demography is linked to selection on quantitative trait Kirkpatrick & Barton, 1997 Trait distribution of populations Ecological optimum trait value Kirkpatrick & Barton (1997). Am Nat., 150, 1-23 Figure adapted from Bridle & Vines (2006). TREE, 22,

8 Demography is linked to selection on quantitative trait Kirkpatrick & Barton, 1997 Stabilizing selection for optimum phenotype Optimum trait changes linearly with habitat θ x = bx Demography linked to deviation from optimum fitness Kirkpatrick & Barton (1997). Am Nat., 150, 1-23 Figure adapted from Bridle & Vines (2006). TREE, 22,

9 Equilibrium 1: Continuous range expansion Populations at equal density across gradient N x = K Each local population at ecological optimum z& x = bx Kirkpatrick & Barton (1997). Am Nat., 150, 1-23 Figure adapted from Bridle & Vines (2006). TREE, 22,

10 Equilibrium 2: Range limit Net gene flow from high to low density populations Flow of maladaptive alleles to peripheral populations Peripheral populations act as a demographic sink Kirkpatrick & Barton (1997). Am Nat., 150, 1-23 Figure adapted from Bridle & Vines (2006). TREE, 22,

11 Model predictions Gradient/dispersal B = Range limit Extinction Range expansion A = Genetic potential Figure adapted from Kirkpatrick & Barton (1997). Am Nat., 150, 1-23

12 Model predictions Gradient/dispersal B = Range limit Extinction Range expansion Range limit occurs when Steeper ecological gradient Greater dispersal kernel Range expansion is easier with Increasing stabilizing selection Increasing heritability A = Genetic potential Figure adapted from Kirkpatrick & Barton (1997). Am Nat., 150, 1-23

13 Model assumptions Gradient/dispersal B = Range limit Extinction Range expansion Environments constant with time Large local population size Constant genetic variance A = Genetic potential Figure adapted from Kirkpatrick & Barton (1997). Am Nat., 150, 1-23

14 Selection, gene flow (+ genetic variation and drift) Barton 2001; Polechova & Barton 2016 Adds the evolution of genetic variance and effects of drift Migration among populations with different trait means = V G Barton (2001). Integrating Ecology and Evolution over spatial contexts

15 Selection, gene flow (+ genetic variation and drift) Barton 2001; Polechova & Barton 2016 Adds the evolution of genetic variance and effects of drift Migration among populations with different trait means = V G With drift, however, V G and sharp range limits form: B > Nσ s Effective environmental gradient i.e. fitness cost of dispersal Efficacy of selection relative to drift Nσ = population size (N) within dispersal kernel (σ) s = strength of selection per locus Polechova & Barton (2014). PNAS, 112,

16 Gene flow and elevational range limit of M. laciniatus Sampled populations from elevational range Genomic DNA from leaf tissue Figure adapted from Sexton et al. (2016). Molec Ecol, 25,

17 Gene flow and elevational range limit of M. laciniatus Sampled populations from elevational range Genomic DNA from leaf tissue Graph-theory approach to estimate gene flow IBD = isolation by distance IBE = isolation by environment Figure adapted from Sexton et al. (2016). Molec Ecol, 25,

18 Lack of evidence for gene flow limiting species range Lack of evidence for drift and swamping gene flow from central populations Abundance and genetic variation similar across range No directional gene flow between central and edge populations Gene flow was by IBE not IBD Figure adapted from Sexton et al. (2016). Molec Ecol, 25,

19 Theoretical models of range limits Models at different types of range margins 1. Abiotic environment 2. Competitive margins 3. Hybridization at margins

20 Modelling selection, gene flow, multispecies competition Case & Taper 2000 Species can be excluded from the range of others due to competition Adaptation is required at range margin in the presence of another species As before, gene flow and low population density may limit adaptation at range edge Darwin (1859). Origin of Species by Natural Selection Case & Taper (2000). Am Nat, 155,

21 Selection, gene flow, multispecies competition Gradient/dispersal C = disruptive: stabilizing Range limit Predictions: Competition makes range limits easier to achieve B = A = Genetic potential Figure adapted from Case & Taper (2000). Am Nat, 155,

22 Model predictions C = disruptive: stabilizing selection High Low Predictions: Gradient/dispersal Range limit Competition makes range limits easier to achieve Range limits break down when: Disruptive > stabilizing (low C) B = A = Genetic potential This divergence will enable species to spread into full sympatry Figure adapted from Case & Taper (2000). Am Nat, 155,

23 Character displacement may lead to range limits Species A Species coexistence will require character displacement Phenotype Species B but character displacement itself may also lead to evolution of range limits Evidence is limited in nature Space Figure adapted from Case & Taper (2000). Am Nat, 155,

24 Theoretical models of range limits Models at different types of range margins 1. Abiotic environment 2. Competitive margins 3. Hybridization at margins

25 Hybridization effects on range limits Hybridization In addition to migration load, there is genetic load due to low hybrid fitness Linkage disequilibria may hamper fixation of locally beneficial alleles Increase likelihood of range limit occurring Goldberg & Lande (2006). Evolution, 60,

26 Hybridization effects on range limits Hybridization In addition to migration load, there is genetic load due to low hybrid fitness Linkage disequilibria may hamper fixation of locally beneficial alleles Increase likelihood of range limit occurring Goldberg & Lande 2006 Low rate of interspecific mating sharp range limit Evolution of hybridization avoidance and character displacement range expansion Goldberg & Lande (2006). Evolution, 60,

27 Summary Since Kirkpatrick & Barton 1997, many models have tested the evolution of range limits Theory predicts importance of maladaptive gene flow, drift, and biotic interactions in setting range limits Empirical evidence testing theories on adaptation at range limits is limited Understanding range limits has many evolutionary and ecological implications